A single switching frequency has commonly been used in power converters, which causes a switched mode power supply (SMPS) to output a periodic ripple voltage. If such an output voltage is used as a supply voltage to a saturated power amplifier, this ripple may mix with the radio frequency carrier and generate spurious side band signals. In a general case, it may already interfere at radio frequency hardware level in cellular phone transmitter applications.
In order to achieve full efficiency benefit when switched mode power supply is used but to reduce spurs in a power amplifier's output at the same time, spreading spurious energy over a wider bandwidth is a way to stay inside system specification limits. Another way is to increase switching frequency considerably but then efficiency would be inadequate because switching big power switches at a high frequency decreases efficiency (dynamic power consumption part increases in a linear manner). When a power amplifier is powered, for instance, from boosted supply voltage (boost converter's supply voltage is taken from a battery) via a controlled buck (step-down) converter, then a frequency increase is not a viable solution since any voltage increment squares power consumption. It is also possible to use external filtering to reduce ripple voltage but this is, of course, costly.
A few basic methods for achieving some sort of spreading are known. In the first method, a reference current is summed with some constant offset. This can be done via an external setting resistor, such as in a commercial chip called LM5020.
Another way is to have a very straightforward method of slowly decreasing/increasing reference current (thus frequency) having so repeated cycles over time. Yet another way is to add a low drop-out linear regulator between a battery and a power amplifier for power control. This way the ripple issues may be reduced but it, in turn, causes decreased efficiency and a shorter battery life.
In patent publications Nos. US2007/0047272 A1 and Ser. No. 11/215,622 a more trimmed versions than just decreasing/increasing a frequency are described where a set of predefined frequencies are multiplexed and selected based on a digital PWM (pulse width modulation) counter value. This is done in a microcontroller environment.
A more radio frequency hardware related setup is described in a U.S. Pat. No. 6,980,039 B1 and an application Ser. No. 10/792,486. They describe a randomized frequency control by using one or two DACs (digital-to-analogue converters) to control an amount of current flowing in a reference capacitor, a linear feedback shift register (LFSR, random number generator), a separate LFSR's oscillator circuitry, and a reference capacitor with a current flow control switch. LFSR has a lower clocking frequency than at ramp generator's output. Further, it is described how a dither current is handled by DACs in order not to generate any nonlinearity.